The electrogeochemical exploration method, CHIM, developed over twenty years ago in the former Soviet Union, is claimed to be a means of collecting ions emanating from ore deposits concealed by thick cover (Goldberg et al., 1990). Available treatises on CHIM (the term is an acronym derived from the Russian phrase "Chastichnoe Izvlechennye Metallov", meaning partial extraction of metals) in the English language are limited. Summaries may be found in Shmakin (1985), Bloomstein (1990), and Antropova et al., (1992). The method is based on the premise that an applied electric field will drive ions in the soil into specially designed collector electrodes. Ions accumulate in an electrolyte within the electrode. The electrolyte, typically nitric acid of 2N to 4N concentration, also serves to conduct current from the power source to the soil through a low-permeability membrane of synthetic parchment located at the base of the electrode.
The CHIM technique can best be described as an geoelectrogeochemical sampling method. It uses a DC electrical current to move mobile cations into special fluid-filled cathodes placed spatially on the earth's surface. Cation-collector electrodes have been designed and developed by the United States Geological Survey (USGS) to practise the CHIM method. They are relatively easy to use and clean, hold liquid well, and have a transparent body so that field crews can monitor for leaks or other problems. The cation collectors used for tests conducted at the Kokomo Mine, Russell Gulch, Colo., have an inside diameter of 1.625 inches (4.128 cm) and an operational capacity of 150 ml.
Electrical contact to the ground is made through a disk of artificial parchment (type 1470) manufactured by the James River Paper Co., Parchment, Mich. The parchment disks are cut to fit the lower cap and are held in place by an O-ring seal. In the absence of seal leaks, the electrode will lose about 5-10 ml of electrolyte through the parchment in 24 hours. A 99.6% pure titanium rod, 5 mm in diameter and 20 cm long, is used as the inner working cathode. The electrolyte used as the cation-collecting medium is 4N reagent-grade nitric acid.
The electrical current, generally ranging from 0.1 to 0.5 A, is conducted through the electrode for time intervals of several hours to several days. At the end of this time, the electrolyte and inner electrodes are collected and analyzed for elements of interest. An important aspect of the CHIM method is that it samples only ions mobile in an electric field as opposed to the total quantity of a particular element in the soil near an electrode. Where the mobile ions are related to a geochemical halo developed in cover above a deposit, CHIM samples may provide better definition of the concealed deposit than standard geochemical methods.
David B. Smith et al., in an article entitled "Preliminary Studies of the CHIM Electrogeochemical Method at the Kokomo Mine, Russell Gulch, Colo.", Journal of Geochemical Exploration, 46 (1993), page 257-278, disclose the results of preliminary tests conducted at the Kokomo Mine, using the CHIM electrogeochemical method.
Specifically, the U.S. Geological Survey started a study of the CHIM method by conducting tests over a precious- and base-metal-bearing quartz vein covered with 3 m of colluvial soil and weathered bedrock near the Kokomo Mine, Colo. The tests show that the CHIM method gives better definition of the vein than conventional soil geochemistry based on a total-dissolution technique. The CHIM technique gives reproducible geo-chemical anomaly patterns, but the absolute concentrations depend on local site variability as well as temporal variations. Weak partial dissolutions of soils at the Kokomo Mine by an enzyme leach, a dilute acetic acid leach, and a dilute hydrochloric acid leach show results comparable to those from the CHIM method. This supports the idea that the CHIM technique is essentially a weak in-situ partial extraction involving only ions able to move in a weak electric field.
The technique uses a DC electrical current introduced into the earth to draw mobile cations into specially designed cathodes. Ions collected in this manner constitute a geochemical sample of mobile ions extracted from soil in the vicinity of the electrode. The technique may be thought of as an in-situ partial chemical extraction.
The overall equipment used in the studies at the Kokomo Mine is generally similar to the Russian CHIM equipment in electrical capacity, with the addition of a multichannel, recording, ampere-hour meter. The USGS system has a capacity of 31 channels, 15 kW, 1000 V, and 43 A. The principal components of the system are (1) power source, (2) ampere-hour recorder, (3) current control rheostats, (4) current distribution cables, and (5) cation-collector electrodes (FIG. 2). The power source is a 15 kW diesel motor generator providing AC power to Zonge GGT-25 transmitter. DC power from the GGT=25 transmitter goes to individual anode and cathode ampere-hour sensor units where the power is split into 31 channels. Each of the 62 individual channels may be monitored for current, and the ampere-hours delivered to each channel are recorded. From the ampere-hour recorder, current goes to banks of rheostats that control current to each cation collector, and from the rheostat banks to multiconductor distribution cables. Take outs on the distribution cables then deliver current to individual electrodes. For studies at the Kokomo Mine, only cation collectors were used. The positive current conductor was directly connected to 3 or 4 graphite bars buried in the ground, salted, and watered. These were placed about 100 m from the cathode array.
Many unsolved problems remain not only in the utilization of the CHIM method as an exploration tool, but also in the basic understanding of the electrochemical processes involved. The major issues and problems include the following:
1. Anion collection. Pathfinder elements such as Au, As, and Sb may be present as anionic species in the near-surface environment (Stumm and Morgan, 1981; Mann, 1984; Webster, 1986). Russian literature translated in Bloomstein (1990) mentions briefly the importance of collecting and analyzing anions but does not give any data or case histories.
2. Ion mobility in the vadose zone. The mechanism of ion mobility in an electric field in dilute solution is well understood. However, in the vadose zone, where most CHIM collection occurs, and in the presence of clays and organic matter that absorb ions, the process of ion mobility is not well understood. If relative mobilities in the unsaturated soil are significantly different than those observed in dilute solution, selective collection could require alteration of conventional interpretation of CHIM data.
3. Destabilization processes at the soil-electrode interface. Elements present as either positive or negative complexes present problems for the CHIM technique. Such complexes are not stable over a wide range of pH. At the electrode-soil interface, these complexes may be destabilized and thus prevented from entering the low-pH electrolyte.
4. Problematical analytical methods. The nitric acid solution from a CHIM cation collector poses analytical problems because the sample contains very low concentrations of some important pathfinder cations such as Au in a matrix containing high concentrations of major cations such as Ca, Mg, Na, Fe, etc. As part of our CHIM research, reference samples of the nitric acid solution from the CHIM runs have been prepared for interlaboratory comparisons.
5. CHIM vs. partial extractions. In the study at the Kokomo Mine, certain partial-dissolution techniques gave results at least as good as the more time-consuming and expensive CHIM method. More comparisons need to be made for various types of mineral deposits in a variety of geologic settings to determine if results from the CHIM method can be duplicated with partial-dissolution techniques.